1. Field of the Invention
The invention relates to plasma-sprayed layers of aluminium oxide on a substrate and to a process for producing plasma-sprayed layers of aluminium oxide.
2. Description of the Related Art
Plasma-sprayed layers of aluminium oxide are used to protect components of ships and aircraft from corrosion or wear (Herbert Hermann, Spektrum der Wissenschaft [Spectrum of Science], November 1988, page 102). These mixed aluminium oxide layers have cracks. It is also known to produce aluminium oxide layers by means of the plasma spraying process (S. Jiansirisomboon et al., Journal of the European Ceramic Society 23, (2003) 961-976). These layers likewise have cracks.
It is known to produce plasma-sprayed layers of aluminium oxide from powders on a micrometer scale. The pore sizes may in this case be in the range from 20 to 40 μm.
It is known to produce coatings by means of the plasma process from nanoscale Al2O3/TiO2 powders (U.S. Pat. No. 6,723,387).
For this purpose, it is known to produce nanoscale aluminium oxide powder by means of chemical vapour condensation (CVC). In this case, the precursor, together with a carrier gas stream, is reacted at a hot wall or in a flame reactor (U.S. Pat. No. 6,723,387). The reactors used in that document are described in documents U.S. Pat. No. 5,514,350 and U.S. Pat. No. 5,876,683.
According to U.S. Pat. No. 5,514,350, non-agglomerated nanostructured (n-ceramic) powders are produced from organometallic precursors by the organometallic precursors in a carrier gas stream being thermally decomposed in a hot fluidized-bed reactor. In particular, powders of the formula n-SiCxN4 are obtained from hexamethyldisiloxane. The document does not describe a pyrogenic process in which evaporable metal oxide/metal compounds are hydrolyzed in the hydrogen-oxygen flame to form the pure oxides.
According to U.S. Pat. No. 5,876,683, non-agglomerated oxidic powders are produced in the size of nanoparticles by an evaporated organometallic compound being mixed with an oxygen-containing fuel gas stream, this gas stream being fed to a flame and burnt there, with a pressure of from 1 to 50 mbar being maintained in the reaction chamber.
The non-agglomerated oxides produced can be applied to a cooled roller or, if the flame is configured as a plasma flame, direct to a substrate, with a solid layer being formed on the substrate.
It is also known for nanostructured aluminium oxide which has been produced by means of the CVC method in a hot-wall reactor to be suspended in an organic liquid and this suspension to be processed by means of the plasma spraying process (US 2003/0077398 A1).
It is also known to produce mechanically stable, crack-free layers of aluminium oxide produced by flame spraying in a thickness of from 50 to 1000 μm in order to increase the stability of components made from quartz, such as for example quartz crucibles. Details as to the structure of the Al2O3-layer are not given (WO 02/092525 A1).
It is also known to produce an amorphous material by means of plasma spraying by aluminium oxide mixed with other oxides being fed to the plasma (WO 2004/016821).
The known Al2O3-layers may disadvantageously have defects, such as pores and cracks. This residual porosity may have an adverse effect on the micro-hardness and wear resistance.
The object therefore exists of producing plasma-sprayed layers of aluminium oxide which have a minimum number of defects, such as cracks and pores within the layer, and a high resistance to wear.